In several combustion processes, and in well-known ammonia production processes, a flue gas containing, inter alia, carbon dioxide is formed. This carbon dioxide component is often separated out or captured from the flue gas in one or more absorber systems using a solvent absorber. Typically, the spent carbon dioxide solvent absorber is then regenerated in one or more regeneration columns. In carbon dioxide capture systems of this kind, there is a need to reduce system energy consumption and capital expenditure (“capex”) requirements. For such solvent-based carbon dioxide capture systems, much of the energy consumption and capex are due to the need for solvent absorber regeneration, i.e., carbon dioxide desorption, in regeneration columns. Regeneration columns have high energy consumption demands due to considerable heat requirements from a reboiler or other source for carbon dioxide desorption. Solvents typically used in solvent-based carbon dioxide capture systems include amines and ammonia. For example, in an ammonia production process a solvent-based carbon dioxide capture system known as the Chilled Ammonium Process (CAP) is normally used. In CAP, ammonia is used as the solvent. Other processes including a solvent-based carbon dioxide capture system use different kinds of amines or mixtures thereof.
WO 2006/022885 discloses ultra cleaning of combustion gas including removal of carbon dioxide in an absorbing subsystem with one or more carbon dioxide absorbing stages and a subsequent regeneration subsystem.
The process for urea production is well-known and extensively disclosed in the literature. Different variants of the urea production process exist, but a common feature is that ammonia and carbon dioxide are used as reactants under elevated pressure in a urea synthesis tower or reactor for the production of urea. According thereto, the following two equilibrium reactions take place:2NH3+CO2⇄NH2COONH4(ammonium carbamate)NH2COONH4⇄NH2CONH2(urea)+H2O
After the urea synthesis reaction, urea, carbamate, water and excess ammonia are transported to a distillation reactor from which a solution or slurry of, inter alia, carbamate in equlibrum with ammonia, carbon dioxide and water is recycled back to the urea synthesis reactor for further reaction.
WO 2008/135150 discloses a method for purifying flue gases from incinerators and then producing urea. In this method, ammonia and flue gas containing carbon dioxide is added to a washing device in which carbon dioxide is absorbed. A solution containing (NH4)2CO3 and (NH4)2CO4 is emitted from the washing unit and transported to a stripper in which (NH4)2CO4 is separated and (NH4)2CO3 is subjected to an energy-consuming decomposition to carbon dioxide and ammonia. This carbon dioxide and ammonia is then used in a urea production process.
GB 1 359 715 discloses an integrated process for the production of ammonia and urea including process steps comprising the absorption of carbon dioxide from a concentrated ammonia solution with a view to yielding a concentrated ammonium carbamate solution to be fed to a urea reactor, wherein the carbon dioxide source is a raw synthesis gas containing carbon dioxide and hydrogen.
Methods to reduce energy consumption and capex of solvent-based carbon dioxide capture systems have been developed, but the need for further cost reductions remain.